This invention relates to a human lumbar model structure for simulating a pressure applied to nucleus pulposus in a human lumbar and a method for fabricating the structure.
In order to estimate and evaluate a ride vibration comfort which would be felt by a human being (an occupant, a driver, a passenger, etc.) on a vehicle such as a motor vehicle, bulldozer, a train, a vessel, and a spacecraft travelling with vibration, variation in speed, and/or change of acceleration, sensory tests have only been used. However, the test results widely fluctuate among individuals. Thus, no basis for objective, persuasive, or quantitative evaluation of vehicles in the ride vibration comfort has been established yet.
Long-distance or long-time drivers often suffer from herniation of a lumbar intervertebral disk. According to one of the most convincing theories accepted by many orthopedists, the occurrence and the aggravation of the above-mentioned hernia are correlated with ride vibration discomfort when travelling in motor vehicles.
The lumbar region of the vertebral column of a human body comprises five vertebrae, that is, 1st lumbar vertebra, 2nd lumbar vertebra, 3rd lumbar vertebra, 4h lumbar vertebra, and 5th lumbar vertebra arranged in the order from upside to down side. There is the lumbar intervertebral disk between each two adjacent lumbar vertebrae. The vertebral column has the nucleus pulposus therein. The herniation readily occurs at the lumbar intervertebral disk between 4th and 5th lumbar vertebrae in comparison with the other lumbar intervertebral disks.
In order to make it possible to objectively and quantitatively evaluate ride vibration comfort which is the cause of suffering from herniation of the lumbar intervertebral disk, U.S. Pat. No. 6,311,562 (B1), which was issued on Nov. 6, 2001 to the same inventor of the present invention, discloses a human lumbar model structure for simulating pressure applied to nucleus pulposus in the lumbar spine of a human body.
The present inventor made a sample of the human lumbar model structure according to the embodiment of FIG. 6 in the US patent and subjected to tests. The sample structure will briefly be described with reference to FIG. 1.
The sample of the human lumbar model structure shown in FIG. 1 comprises a cylindrical lumbar assembly and an abdominal/back muscle member. The,lumbar assembly comprises an upper body member 17, an upper lumbar member 11 corresponding to a lumbar vertebra such as the fourth lumbar vertebra, an annulus fibrosus member 13 corresponding to the nucleus puplosus, nucleus pulposus agent or liquid 16 in the annulus fibrosus member 13, a lower lumbar member 14 corresponding to an adjacent lumbar vertebra such as the fifth lumbar vertebra, and a lower body member 18. These members are formed in a cylindrical shape, coaxially arranged around a common center axis and are vertically superposed from top to bottom in this order.
The abdominal/back muscle member shown at 19 is made of an elastic material and has a hollow cylindrical shape. The abdominal/back muscle member 19 surrounds and tightens the lumbar assembly over at least a center area of a narrowed portion thereof extending from the midlevel of the upper body member 17 to the midlevel of the lower body member 18. In detail, the upper body member 17 comprises an upper portion 171 and a narrowed portion 172 as its lower portion with a reduced diameter and has a shoulder 173 on its outer surface at a bound between the narrowed lower portion 172 and the upper portion 171. Similarly, the lower body member 18 comprises a lower portion 181 and a narrowed portion 182 as its upper portion with a reduced diameter and ahs a shoulder 183 on its outer surface at a bound between narrowed upper portion 182 and the lower portion 181. When the abdominal/back muscle member 19 is attached to the lumbar assembly, it tightly surrounds the narrowed portions 172 and 182 and has an inner diameter equal to the diameter of the narrowed portions 172 and 182 but has an outer diameter larger than an outer diameter of the upper portion 171 of the upper body member 17 and the lower portion 181 of the lower body member 18. Thus, the shoulders 173 and 183 serve as stoppers against axial or radial movement of the abdominal/back muscle member 19.
The upper lumbar member 11 comprises an upper element 111, which is formed in a circular columnar block and tightly fitted into a bottom surface of the upper body member 17, and a lower element 112, which is a circular disk shape and threaded into a bottom surface of the upper element 111. And the lower lumbar member 14 comprises an upper element 141 in a ring form and a lower element 142 in a cylindrical form with an upper dosed end. The upper element 141 is threaded onto the upper outer surface of the upper lower element 142. The lower element 142 is tightly fitted into an upper surface of the lower body member 18, and is provided with a pressure sensor 21 as a pressure transducer therein on a common center axis position. A signal cable 23 from the pressure sensor 21 is lead out through a vertical guide hole 22 formed in the lower body member 18. The annulus fibrosus member 13 is made of a full hard and elastic material and has a flexible structure with thickish and annular form. The annulus fibrosus member 13 is disposed between the lower element 112 of the upper lumbar member and the lower element 142 of the lower lumbar member and tightly fitted at opposite ends to the both lower elements 112 and 142. The annulus fibrosus member 13 is disposed in the ring-shape upper element 141 of the lower lumbar member 14. By this structure, an inner cavity is formed in the annulus fibrosus member 13 and is filled with the nucleus pulposus liquid 16 which is liquid as an artificial nucleus pulposus therein.
A loading member modeling the upper half of a human body is arranged on the top surface of the human lumbar model structure composed by fabricating above-mentioned. The ride vibration comfort or discomfort is evaluated by recording results of detecting instantly and continuously changes of the output voltage of the pressure sensor 21 when the human body fluctuates. For this purpose, the completed human lumbar model structure was subjected to a static loading test in which data are recorded about changes of pressure obtained from the nucleus pulposus member 16 by the increase and the decrease of the vertical loading onto the model structure.
However, as results of the static loading experiment, as shown in FIG. 2, we have found out a hysteresis phenomenon and non-linearity between pressure values obtained for loading up gradually and for loading down gradually from the value loaded once. This means that pressure fluctuation characteristic measured is not constant but unstable at different tests. Further, the same pressure value cannot be obtained for the re-assembling model after the model structure was disassembled. This means that the results of the measurements are not constant but unstable at every model structures assembled or reassembled.
Accordingly, we had various experiments so as to search reasons that cause the problems such as the hysteresis phenomenon, non-linearity and unstability. As the result, we found out the followings. When a vertical loading is applied to the human lumbar model structure, the abdominal/back muscle member 19 is distorted so that upper edge and the lower edge of the muscle member 19 go over the shoulders 173 and 183 to move onto the upper portion 171 of the upper body member 17 and the lower potion 181 of the lower body member 18. The edge going-over phenomenon is one of reasons causing the problems, the present inventor considered. There is difference in contact forces with the upper body member 17 and the lower body member 18 and also in the internal stresses between portions of the muscle member 19 remaining on the upper and lower portions 171 and 181 of the upper and the lower body members 17 and 18 and edge portions going over the shoulders 173 and 183 onto the narrowed portions 172 and 182. The internal stress difference or internal stress distribution affects restoration of the muscle member 19 when the muscle member 19 is relaxed from the loading. Accordingly, there appears the hysteresis between increase and decrease of the loading. Further, an amount of the edge going-over irregularly varies dependent on increase of the loading. Accordingly, the hysteresis is not constant over a range of loading varied. Accordingly, the restoration is not linear over the range of loading varied. Further, the amount of the edge going-over changes in tests repeated. Accordingly, the static lading characteristic is not constant in tests repeated. This means that the test results using the model structure are unstable.
In the fabrication processes of the model structure, the annulus fibrosus member 13 is put on the lower element 142 and in the upper ring element 141 of the lower lumbar member 14. The nucleus pulposus liquid 16 such as grease is filled in the inner cavity of the annulus fibrosus member 13, and then the lower element 112 of the upper lumbar member 11 is fit and tightened onto an upper portion of the annulus fibrosus member 13. In this fabricating process, no reappearance is not insured in the pressure of the nucleus pulposus liquid 16 filled in the inner cavity of the annulus fibrosus member 13. Further, any gas and/or liquid may possibly mixed in the nucleus pulposus liquid 16. These cause unstability that measurement values of the pressure variation change in every model structures assembled or reassembled, the present inventor considers.
It is therefore an object of the present invention to provide a human lumbar model structure which can obtain a stable pressure variation characteristic in a static loading test and stably measuring pressure changes applied to the nucleus pulposus agent of an artificial nucleus pulposus.
It is another object to provide a method for fabricating the human lumbar model structure.
According to this invention, a human lumbar model structure for simulating pressure applied to nucleus pulposus in the lumbar spine of a human body is obtained. The structure comprises a cylindrical lumbar assembly having an upper body member, an upper lumbar member, an annulus fibrosus member, a lower lumbar member, and a lower body member, which are coaxially arranged around a common center axis and which are superposed vertically from top to bottom in this order. The upper body member comprises a first upper portion having a first diameter and a first lower portion having a second diameter smaller than the first diameter to form a first shoulder portion around the bound between the first upper portion and the first lower portion. The lower body member comprises a second upper portion having a third diameter and a second lower portion having a fourth diameter larger than the third diameter to form a second shoulder portion around the bound between the second upper portion and the second lower portion. The annulus fibrosus member comprises a thick cylindrical ring made of a hard elastic material and having upper and lower parts fitted into the upper and lower lumbar members, respectively. The fibrosus member has an inner cavity filled with a liquid the nucleus pulposus. An abdominal/back muscle member tightly surrounds the lumbar assembly and is made of an elastic material in a hollow cylindrical shape. The abdominal/back muscle member has an upper and lower cylindrical portion fitted onto the first lower portion of the upper body member and the second upper portion of the lower body member, respectively, so that the abdominal/back muscle surrounds and tightens the lumbar assembly over a center area extending from the first shoulder portion of the upper body member to the second shoulder portion of the lower body member, the abdominal/back muscle member having an outer diameter smaller than the first diameter and the fourth diameter when no vertical loading is applied to the lumbar model structure. A pressure sensor as a pressure transducer is provided to be arranged along a center axis of the lower lumber member in direct contact with the liquid as the nucleus pulposus for detecting the variation of the pressure of the liquid in the fibrous member.
According to the human lumbar model structure, the first and second shoulder portions form the hood-like radial flanges radially outwardly protruding over the outer edges of the abdominal/back muscle member to prevent the upper and lower edges of the abdominal/back muscle member from going over the first and second shoulders to move onto the upper portion of the upper body member and the lower portion of the lower body member.
Accordingly, the human lumbar model structure of this invention can provides stabilized static loading characteristic with linearity and no hysteresis.
According to another aspect of this invention, the human lumbar model structure has a penetration path in the lower element of the upper lumbar member, the penetration path upwardly extends from the inner cavity of the annulus fibrosus member in the axial direction to open in a top surface of the lower element of the upper lumbar member. In the fabrication of the human lumbar model structure, the nucleus pulposus liquid in the cavity can overflow the upper opening of the penetration path and can reach a stable condition having a predetermined pressure value. Accordingly, the human lumbar model structure can provide test data stabilized.
The human lumbar model structure according to this invention can be fabricated by the following method, which comprises steps of:
tightly fitting the lower lumbar member with the pressure sensor into an upper surface of the second upper portion of the lower body member;
tightly fitting the annulus fibrosus member to the lower lumbar member;
filling the liquid as the nucleus pulposus into the inner cavity thereof;
tightly fitting the lower element of the upper lumbar member to the upper portion of the annulus fibrosus member;
making the liquid overflow from an upper opening of the penetration path of the lower element;
tightly fitting the upper element of the upper lumbar member to the lower element of the upper lumbar member;
tightly fitting the lower portion of the abdominal/back muscle member onto the second upper portion of the lower body member; and
tightly fitting the upper body member to the upper element of the upper lumbar member, while the first lower portion of the upper body member being fitted into the upper portion of the abdominal/back muscle member.